# std::basic_regex<CharT,Traits>::mark_count

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< cpp‎ | regex‎ | basic regex

unsigned mark_count() const; |  | (since C++11)

Returns the number of marked sub-expressions (also known as capture groups) within the regular expression.

### Parameters

(none)

### Return value

The number of marked sub-expressions within the regular expression.

### Exceptions

May throw implementation-defined exceptions.

### Example

Run this code

    #include <iostream>
    #include <regex>

    int main()
    {
        std::regex r1{"abcde"};
        std::cout << "r1 has " << r1.mark_count() << " subexpressions" << '\n';
        // Expected: 0

        std::regex r2{"ab(c)de"};
        std::cout << "r2 has " << r2.mark_count() << " subexpressions" << '\n';
        // Expected: 1

        std::regex r3{"a(bc)d(e)"};
        std::cout << "r3 has " << r3.mark_count() << " subexpressions" << '\n';
        // Expected: 2

        // Nested sub-expressions
        std::regex r4{"abc(de(fg))"};
        std::cout << "r4 has " << r4.mark_count() << " subexpressions" << '\n';
        // Expected: 2

        // Escaped parentheses
        std::regex r5{"a(bc\\(\\)de)"};
        std::cout << "r5 has " << r5.mark_count() << " subexpressions" << '\n';
        // Expected: 1

        // Using nosubs flag
        std::regex r6{"ab(c)de", std::regex_constants::nosubs};
        std::cout << "r6 has " << r6.mark_count() << " subexpressions" << '\n';
        // Expected: 0
    }

Output:

    r1 has 0 subexpressions
    r2 has 1 subexpressions
    r3 has 2 subexpressions
    r4 has 2 subexpressions
    r5 has 1 subexpressions
    r6 has 0 subexpressions

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